The Top Java Servlets Interview Questions for Aspiring Developers

Java servlets are a fundamental technology for building web applications in Java. As an aspiring Java developer, you can expect to encounter servlet interview questions during the hiring process. In this comprehensive guide, I will provide an overview of servlets and share the most common servlet interview questions and answers to help you ace your next interview.

What are Java Servlets?

Java servlets are classes that dynamically process requests and construct responses. They run on a servlet container or application server which handles the HTTP request/response and lifecycle management. Servlets provide a component-based model for building web applications handling the business logic and interacting with databases and other services.

Some key characteristics of servlets:

• They implement the javax.servlet.Servlet interface.
• They extend the javax.servlet.GenericServlet or javax.servlet.http.HttpServlet base classes.
• The servlet container handles threading and invocation. Servlets themselves are not multithreaded.
• They have access to request and response objects.
• They can forward requests to other resources such as JSPs.
• They are faster and more powerful than traditional CGI scripts.

Now that we’ve covered the basics, let’s look at some common servlet interview questions and answers.

Top Servlet Interview Questions and Answers

Q1. What is the difference between a web server and application server?

A web server (like Apache) handles HTTP requests and serves static content. An application server (like Tomcat) can execute application logic through technologies like servlets and JSPs. The application server sits on top of the web server and enhances it with dynamic capabilities.

Q2. What are some advantages of servlets over CGI scripts?

Some advantages include:

• Better performance – servlets execute in threads rather than processes. Less memory overhead.
• Platform independence – servlets use Java’s “write once, run anywhere” principle.
• Robustness – the servlet container handles concerns like threading, connection pooling, memory management etc. Developers just focus on business logic.
• Maintainability – servlets have a clean component based structure. Easy to modify and re-use code.

Q3. What are the key components of the servlet architecture?

The core components are:

• Servlet interface – defines lifecycle methods like init, service, destroy etc.
• ServletRequest – encapsulates the client request.
• ServletResponse – encapsulates the response back to client.
• ServletConfig – initialization parameters for the servlet.
• ServletContext – web application scope information.

Q4. Explain the servlet lifecycle.

The lifecycle consists of the following phases:

1. Load servlet class and create instance.
2. Invoke init() method.
3. Handle client requests via service() method.
4. Invoke destroy() method before removing servlet from service.

Q5. What is the difference between generic servlets and HTTP servlets?

GenericServlet is protocol independent. HttpServlet extends it with HTTP specific methods like doGet, doPost etc. Normally we would extend HttpServlet for building web apps over HTTP.

Q6. How can servlets communicate with each other?

Servlets can use the following methods:

• Request dispatcher forward() and include() methods
• ServletContext attributes
• HttpSession attributes

Q7. How does session management work in servlets?

Some ways to manage user sessions are:

• Cookies – small data sent from server and stored on client.
• Hidden form fields – session ID passed back and forth as a hidden field.
• URL rewriting – session ID appended to URLs linking to next pages.
• HttpSession API – associate session data as key-value pairs.

Q8. What is the difference between getRequestDispatcher() and sendRedirect()?

• getRequestDispatcher() returns a RequestDispatcher object which can forward() a request to another resource in the same application.
• sendRedirect() sends an HTTP redirect response to the browser directing it to a different URL. This will be a completely new request.

Q9. How do filters work in servlets? What are they used for?

Filters implement the javax.servlet.Filter interface. They wrap around a servlet and preprocess requests and postprocess responses. Common uses include logging, auditing, data compression, JWT authentication etc.

Filters configured for a servlet are invoked before dispatching request to the servlet and after getting response from servlet.

Q10. What are some important servlet annotations?

Some annotations are:

• @WebServlet – defines a servlet class
• @WebFilter – defines a filter
• @WebListener – defines a listener class
• @WebInitParam – initializes init parameters

Q11. How does multithreading work in servlets?

The servlet container handles multithreading. Each request is served by a separate thread from a pool. So we don’t have to worry about thread creation or handling concurrent requests in the servlet class.

However, we still need to synchronize access to any shared data used across different requests. The servlet class itself is not multithreaded.

Q12. What are some benefits of asynchronous servlets?

Asynchronous servlets allow blocking operations like database/network I/O to be handled asynchronously using callbacks. Some benefits are:

• Threads are not blocked
• Better scalability under heavy loads
• Use callbacks for push notifications

Q13. How can you upload files in a servlet?

We can get access to uploaded files in a servlet through the HttpServletRequest object. Key steps are:

• In HTML form specify method="post" and enctype="multipart/form-data”.
• In servlet use request.getPart() or request.getParts() to process uploaded files.

Q14. What are some differences between servlets 2.5 and servlets 3.0?

Some differences are:

• Annotation based configuration, no need for web.xml
• Asynchronous servlet support
• HTTP protocol upgrade mechanism
• Enhancements to fragment and dependency management

Q15. How can you achieve transport layer security for a web application?

We can use HTTPS and SSL certificates to add transport layer security. The application server should be configured with a digital certificate issued by a trusted Certificate Authority (CA). Access to application is over HTTPS, and all data is encrypted on the network.

Servlets are at the heart of Java web application development. I hope these common servlet interview questions and answers help you in preparing for your next Java interview. The key is to understand the servlet architecture, lifecycle, config models, and be able to discuss alternatives for things like session management, security etc.

Make sure you are familiar with the servlet API classes like HttpServletRequest, HttpServletResponse, ServletContext etc. Brush up on your knowledge of related topics like JSP, filters, listeners and asynchronous processing support.

With a strong grasp of servlet fundamentals, you will be able to answer the interview questions confidently and land your dream Java web developer job!